There's a lot of work to be done. Rising housing costs have pushed many New Yorkers far from existing subway lines, and there's a strong argument that expanding the underground system is not a viable solution.

The Pratt Center for Community Development works with small businesses, policymakers, and other organizations to make New York more sustainable and equitable. In a recent report funded by the Rockefeller Foundation, itmakes a convincing case for expanding a system known as Bus Rapid Transit.

It's a three part argument.

1. Expanding the subway system is prohibitively expensive and slow.

The report doesn't mince words here: "There is no imaginable scenario in which the subway system could be expanded to meet the needs of fast-growing outer borough neighborhoods."

There are two projects already underway to make minor additions to the current subway network, and both have taken years and cost billions of dollars. The extension of the 7 line to the West Side, if completed on time next summer, will have taken five years and $2 billion — to add just one station to the MTA's map.

New York has been trying to build a new subway under Second Avenue since the 1920s. The first three new stations — Phase 1 of the project — should finally open in December 2016, for a price of $4.45 billion.

Lower property values and less density would presumably make building new subways outside Manhattan more affordable, but probably not by enough to make other new lines or extensions affordable for the eternally cash-strapped MTA. And even then, it takes years and years to do all the planning, digging, and building.

2. Bus Rapid Transit is cost-effective and offers many of the benefits of a subway line.

Bus Rapid Transit (BRT) is a bus line with some elements of a subway. The buses have their own lanes and get traffic signal priority at intersections, so they move quickly. They don't stop that often (and don't replace local bus service), so they cover large distances efficiently. Riders pay their fares at stations, so the bus isn't stuck waiting for people to pay as they board.

They would operate on large boulevards, where losing one lane of traffic wouldn't slow everything down. The lanes require some construction (either new paint or physical medians, along with small stations), but that pales in comparison to what a subway line needs.

Plus, buses are more resilient than buses, since they don't require tunnels that can flood during major storms like Hurricane Sandy.

3. New York really needs to expand its public transportation system.

Recent changes in where New Yorkers work and live make clear the need for an expanded transit system. Here's where the maps come in.

Let's start with a look at population change in New York over the past two decades. Rising housing costs in Manhattan and other areas have pushed many low-income New Yorkers far from the subway system. You can see there are lots of areas in dark blue — those with the fastest growth rate — quite far from existing subway lines.

That means a lot of households that depend on multiple wage earners are also far from the subways. According to the Pratt report, more than 758,000 New Yorkers commute over an hour each way. That's a lot of time wasted, especially since two-thirds of those commuters earn less than $35,000 per year.